Therefore, this double-blind randomized controlled trial will assess the benefit of deworming (mebendazole), integrated into routine child health care visits in a highly STH-endemic area (Iquitos, Peru), on the primary outcome of weight gain. Timing, frequency and impact of deworming will be considered. A total of 1760 children will be recruited at their routine 12-month check-up visit and randomly assigned to one of four intervention groups: Group 1 will receive usual care and mebendazole (single dose 500 mg) at their 12-month visit and usual care and a placebo tablet at their 18-month visit; Group 2 will receive usual care and a placebo tablet at their 12-month visit and usual care and mebendazole at their 18-month visit; Group 3 will receive usual care and mebendazole at both their 12-month and 18-month visit; and Group 4 will receive usual care and placebo at both their 12-month and 18-month visit. Usual care will consist of age-appropriate immunizations, supplements and other Peruvian Ministry of Health-recommended interventions. All children will be followed up to their 24-month visit and all will be given mebendazole at that time.